The iPhone 5S, the Moto X, and the rise of the co-processor

Over the last few years, chipmakers have relied more and more on integration to save energy, use less physical space, and increase performance in their products. It started with little things like the memory controller, but we have reached the point where almost every important chip in a computer can be combined into one do-everything piece of silicon called a "system-on-a-chip" (or SoC for short).

This trend isn't going anywhere, but a few high-end phones recently have been trying something a bit different to enable unique features—they've included one (or more) small, low-power, extremely specialized co-processors designed to support very specific features. Why do this instead of building those features into the SoC along with everything else? We'll look at the just-announced iPhone 5S and the recently released Moto X to explain why.

Saving power

The M7 co-processor in the 5S and the pair of co-processors in the Moto X's "X8 Computing System" have one big thing in common: they're designed to do their thing when the phone is idle, or mostly idle. The M7 receives and processes data from the iPhone's various motion sensors, even when the phone is off and in your pocket, and one of the two co-processors in the Moto X is designed to show the low-power Active Notifications when the phone moves; the other is always listening for voice input.

So why not integrate these features into the SoC itself? If you know anything about SoC design, you probably realize that the chips contain many things besides the CPU and GPU, and some of these blocks are actually more-or-less equivalent to co-processors. Video encoding and decoding, for example, is generally handled by a small dedicated block so that you can play video without taxing the more power-hungry CPU or GPU.

That's fine for tasks like video decoding that occur often but not always, but not so much for features that are running constantly. While modern SoCs are generally good at "power gating," or shutting off unused portions of the chip when they're not needed, keeping any part of the SoC enabled at all times will lead to small amounts of power leakage. Over time, even this minor leakage can have an adverse effect on battery life.

None of this is to say that using a co-processor is always the right way to go, but it makes sense for always-on-in-the-background features like those sported by the iPhone 5S and Moto X. Phones and tablets always have to maintain a delicate balance between performance, features, and power usage, and even small adjustments, like using a co-processor instead of the main SoC, can result in better battery life over time.

Staying flexible

Enlarge/ The iPhone 5S is the first Apple device to use the M7, but it almost certainly won't be the last.

Andrew Cunningham

Using co-processors to enable this unique functionality also gives some flexibility to Apple, Google, and anyone else who wants to enable similar features.

Take the X8 Computing System (please!): in the Moto X, it consists of a Qualcomm Snapdragon SoC and the two co-processors, but Google has gone on the record that the co-processors are in no way married to that particular SoC. If Google and Motorola want to create a lower-cost version of the Moto X with a lesser SoC or upgrade the Moto X next year, they are free to use whatever chip they want to power most of the phone's operations. Because co-processors handle the Active Notifications and the touchless controls, Motorola won't have to throw the baby out with the bath water every time it changes SoCs.

While Apple doesn't talk about its future plans, I'm pretty sure that the iPhone 5S won't be the only iDevice that picks up an M7. Say Apple puts out a new iPod touch next year (and it does seem to have settled into a two-year refresh cycle for that particular hardware, barring some kind of October surprise) and wants to increase its utility as a fitness device. Apple wants to increase its performance, but doesn't want to put a top-end A7 or A8 into a cheaper, lower-margin device. Keeping the M7 separate from the SoC would give the iPod maker the freedom to pair this hypothetical sixth-generation Touch with an A6, enabling those always-on features, but again not tying them to any specific chip.

As others have noted, the M7 also seems like an ideal candidate for some kind of wearable computing device, where battery life is already shaping up to be an even bigger concern than it is in smartphones. Imagine the M7 paired with something like that single-core A5 that showed up in the Apple TV earlier this year, and you've got some pretty convincing smartwatch guts (assuming Apple is indeed working on such a device).

Computing devices, and the chips that power them, are going to continue to become more integrated—there's no question about that. However, there are certain kinds of features where breaking something out from a monolithic SoC still makes sense, and as more and more manufacturers jump on the always-on-feature train, we're only going to see more of them.